This invention relates to improving the production of fluids from wells penetrating subterranean formations. More specifically it relates to a method for increasing the ability of fractures to drain formations. In particular it relates to propped fractures, that have wormholes extending from the faces of the fractures into the formation, and methods of creating such fractures.
The flow of fluids through porous media, for example the production of fluids from wells, is governed by three principle factors: the size of the flow path, the permeability of the flow path, and the driving force.
It is often necessary to stimulate the production of fluids from subterranean formations when wells are not producing satisfactorily. The failure to produce is typically due to an inadequate, or a damaged, path for fluids to flow from the formation to the wellbore. This may be because the formation inherently has insufficient porosity and/or permeability, or because the porosity and/or permeability have been decreased (damaged) near the wellbore during drilling and/or completion and/or production. There are two main stimulation techniques: matrix stimulation and fracturing. Matrix stimulation is accomplished by injecting a fluid (e.g., acid or solvent) to dissolve and/or disperse materials that impair well production in sandstones or to create new, unimpaired flow channels between the wellbore and a carbonate formation. In matrix stimulation, fluids are injected below the fracturing pressure of the formation. Matrix stimulation, typically called matrix acidizing when the stimulation fluid is an acid, generally is used to treat only the near-wellbore region. In a matrix acidizing treatment, the acid used (typically hydrochloric acid for carbonates) is injected at a pressure low enough to prevent formation fracturing. It is desirable to take into account well and formation factors (such as temperature and formation composition) and adjust treatment parameters (such as acid strength and injection rate) so that dominant “wormholes” are formed which penetrate through the near wellbore area.
When acid is pumped into a formation, such as a carbonate (limestone or dolomite) formation, at pressures below the fracture pressure, the acid flows preferentially into the highest solubility or the highest permeability regions (that is, largest pores, vugs or natural fractures). Acid reaction in the high-solubility or high-permeability region ideally causes the formation of large, highly conductive flow channels called wormholes that form approximately normal to the fracture. The creation of wormholes is related to the rate of chemical reaction of the acid with the rock. High reaction rates, as observed between typical concentrations of unaltered mineral acids, such as HCl, and carbonates, tend to favor wormhole formation. Acids normally used in field treatments are highly reactive at reservoir conditions and tend to form a limited number of wormholes. A low reaction rate favors the formation of several small-diameter wormholes. However, unless the treatment is designed properly, wormholes are not formed. Instead, for example if the acid flux is too low, the acid reacts evenly with the formation, which is commonly called compact dissolution, dissolving all the rock near the wellbore and not penetrating deep into the formation and creating flow paths there. Wormholing is desirable in matrix acidizing.
In fracturing, on the other hand, a fluid is forced into the formation at a pressure above that at which the formation rock will part. This creates a greatly enlarged flow path. However, when the pressure is released, the fracture typically closes and the new flow path is not maintained unless the operator provides some mechanism by which the fracture could be held open. There are two common ways of doing this. In conventional propped hydraulic fracturing, the fluid that is used to generate or propagate the fracture is viscous and carries a solid proppant that is trapped in the fracture when the pressure is released, preventing the fracture from closing. In acid fracturing, also known as fracture acidizing, the fracture is generated or subsequently treated with an acid. In this case, however, the treatment parameters have in the past been adjusted so that wormholing did not occur. Instead, the object previously has been to etch the faces of the fracture differentially. Then, when the pressure is released, the fracture does not close completely because the differential etching has created an asperity between the faces so that they no longer match up and there are gaps where material has been removed. Ideally the differential etching forms flow channels, usually generally running along the faces of the fracture from the wellbore to the tip, that enhance production. In acid fracturing, wormholing was undesirable because in methods used previously it does not occur at many points along the fracture but rather primarily occurs only where the acid most easily or first contacts the formation. This is most typically near the wellbore, although if there are natural high-conductivity streaks, fissures, vugs, etc., there could be other locations with a high intensity of wormholes. This increases the amount of acid required (wastes acid that would otherwise be used to etch the conductive channels) and increases the pump rates required to propagate the fracture and keep the fracture open. Thus when there are wormholes near the wellbore in acid fracturing, large amounts of acid and high pump rates are required so that the fluid that reaches far out into the fracture, if a fracture can be formed at all, is still sufficiently acidic to react with the fracture faces. This situation is exacerbated by the fact that, even though the pump rate as seen at the wellhead can be high, the fluid velocity out in the fracture (affecting the rate at which fresh acid reaches that point) can be very low because the surface area of the fracture faces increases greatly as the fracture is propagating.
In production from a fracture-stimulated well, the extent of the available flowpath is a function of the size and shape of the fracture, and in particular of the effective surface area of the faces of the fracture. The permeability of the flowpath is the effective permeability of the fracture after closure, that is, the effective permeability of the proppant pack or of the etched channels. The driving force is the pressure differential between the fluid in the formation and the fluid in the wellbore. This driving force varies along the length of the fracture. The optimal fracture would be one with a large effective surface area and a high effective permeability. As it relates to maximizing production, this would be the equivalent of having a larger effective wellbore radius. It would therefore require only a small pressure drop to provide a high fluid flow rate out of the formation and into the wellbore.
In the past, the only way to generate a fracture with a high effective surface area for flow of fluids from the formation into the fracture was to generate a fracture that was either high (assuming a vertical fracture) or long (extending far from the borehole) or both, and the best way to generate a fracture having a high effective permeability was with proppant. Propped fractures having wormholes extending from their faces out into the formation, and methods of forming such fractures, would be highly desirable because they would have high effective surface areas and the wells would have high effective wellbore radii.
U.S. Pat. No. 3,768,564 discloses a process wherein unpropped fractures are allowed to close prior to prolonged contact with acid. Flow channels are etched while the fracture is held open, then expanded only after the fracture is allowed to close. U.S. Pat. No. 3,842,911 describes the use of propping agents in this process. It describes the formation of a fracture and the introduction of propping agent into the fracture, followed by the complete closure of the fracture on the propping agent and then injection of acid under conditions at which the fracture remains closed, allowing creation of flow channels a relatively long distance from the wellbore. U.S. Pat. No. 4,245,702 describes a process of fracturing and acidizing a well with the use of propping agents that is particularly applicable to relatively hard formations. U.S. Pat. No. 3,642,068 describes the creation of a fracture by means of a viscous medium followed by the passage of propping agents into the fracture. The agent is shifted to a remote location in the fracture by means of an acid that etches those parts of the fracture walls that are close to the borehole. Subsequently the fracture is closed. Formation of wormholes is not proposed in any of these fracturing methods.